>> reporter: a suspect was arrested and tried, but then acquitted, the evidence was sparse, the witnesses willing to testify, few, even though terrell was shot in a church parking lot on a busy street. >> people were out there, but nobody said anything, yes, people coming home from work they were driving back you had to see somebody running down the street with a gun and shooting it. >> reporter: in many of these murders, a code of silence descends like a burial shroud. tom byrne is chicago's chief of detectives. >> people don't want to cooperate, and don't, violate the snitch code, people who live in the community, the good people they may be fearful. >> reporter: 87 percent of the murders in chicago involve shootings and they can be especially hard to solve. >> all we may have would be a casing and unfortunately a blood swab from a victim and that's it. >> reporter: 20 years ago, chicago's homicide rate was much higher, but the arrest rate was as well. 60 percent in 1992.